Under your NWN2 page in GOG Galaxy, you need to click the icon in the top right and go under "Manage Installation" to "Configure" (like so):
Once you're in that page, you can select which version you'd like (you may have to "show more versions" to see it), and you'll simply select v1.23, the last official update to the game from Obsidian, like so:
Then simply click "OK," and your GOG Galaxy launcher should download the required files to revert your game to the original, working version.
It's worth noting that this bug will NOT affect people who downloaded offline installers prior to 2 months ago, but WILL affect anyone who downloads offline installers moving forward. The offline installers don't provide a regular 1.23 version anymore, so for now the only way to fix this that I'm aware of is using GOG Galaxy launcher to downgrade the game. Once the game is downgraded, I imagine you can even uninstall the launcher if you don't like it, but don't quote me on that.
It will also NOT affect people who are playing in exclusive fullscreen, and only happens if you play the game windowed (or use the CE for borderless window).
Hope this helps someone, it took me hours to figure out why I was having this problem and I've seen many others with it so far as well. Thanks for coming to my TED Talk.
